Method for fabricating read only memory

ABSTRACT

A fabrication method for a read only memory provides a substrate having a memory cell region and a periphery circuit region. A memory cell region has a memory cell array and the periphery circuit region has transistors. A precise layer having a plurality of first openings is formed in the memory cell region. The first openings are above the channel region of each memory cell in the memory cell array and the critical dimension of the first openings is identical. A mask layer having second openings and third openings is formed on the substrate. The second openings locate over a pre-coding memory cell region, and the third openings locate over the transistor gates. An ion implantation is performed to code the memory cell in the pre-coding memory cell region and to adjust the threshold voltage of the transistor, using the precise layer and the mask layer as a mask.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of Taiwanapplication serial no. 91 105278, filed on Mar. 20, 2002.

BACKGROUND OF INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for fabricating amemory device. More particularly, the present invention relates to amethod for fabricating a read only memory device.

[0004] 2. Description of Related Art

[0005] The read only memory device provides the non-volatile property,wherein the stored information is retained even electrical power isinterrupted. As a result, the read only memory device is incorporatedinto many electronic products to maintain a normal operation for theproducts. The mask read only memory device is the most fundamental typeof read only memory device. A typical mask ROM device uses a channeltransistor as the memory device. The programming of a mask ROM device isaccomplished by selectively implanting ions to an identified channelregion. By altering the threshold voltage, the control of the “on” stateand the “off” state of the memory device is thus achieved.

[0006] A typical mask read only memory device comprises a polysiliconword line crossing over a bit line. The region under the word line andbetween the bit lines is the channel region of the memory device.Whether or not ions are implanted to the channel region determines thestorage of the binary digit of either “0” or “1”, wherein the implantingof ions to the identified channel region is known as code implantation.

[0007] In general, when a code implantation process is performed on amask read only memory, a patterned photoresist layer that exposes apre-coding region is formed with a photomask. An ion implantationprocess is then performed to implant dopants to the pre-coding region inthe substrate under the stacked gate structure. Usually, as required bythe circuit design, an isolated pattern region and a dense patternregion are formed on a same photomask. An overexposure and anunderexposure in the isolated pattern region and in the dense patternregion often occur during the transferring of a mask pattern due to theoptical proximity effect. The accuracy of the critical dimension of thetransferred pattern is thereby affected. Consequently, a misalignmentwould result as dopants are being implanted to the identified channelregion of a mask read only memory device during the code implantationprocess. Information that is being stored in the memory cells of thememory device is thus erroneous to adversely affect the operationalfunction of the memory device and the reliability of the products.

[0008] Conventionally, to correct the non-uniformity of pattern in thedense pattern region and the isolated pattern region of a coding mask ofa mask read only memory device, the optical proximity correctiontechnique is employed to design a mask with a special pattern. However,this type of mask with a special pattern, not only increases themanufacturing time, it also complicates the manufacturing process andincreases the manufacturing cost. Further, after the fabrication of thephotomask is completed, it is rather difficult to debug the defects inthe pattern of this type of photomask.

SUMMARY OF INVENTION

[0009] Accordingly, the present invention provides a fabrication methodfor a read only memory device, wherein the openings in the dense patternregion and in the isolated patter region in the memory cell array of themask read only memory device are formed having the same dimension, evenwithout the application of the optical proximity correction technique.

[0010] Based on the foregoing reasons, the present invention provides afabrication method for a read only memory device. The method provides asubstrate that comprises a memory cell region and a periphery circuitregion, wherein a memory cell array is formed in the memory cell regionand a plurality of transistors is formed in the periphery circuitregion. A precise layer that comprises a plurality of first openings isthen formed on the identical. Further these first openings are locatedabove the channel region of each of the first memory cells in the memorycell array. A mask layer that comprises a plurality of second openingsand third openings is formed on the substrate. The aforementioned secondopenings are positioned above a plurality of the pre-coding memory cellregions in the memory cell region, while the third openings arepositioned above the gates of the transistors in the periphery circuitregion. An ion implantation process is then conducted to code the memorycells in the pre-coding memory cell region and to adjust the thresholdvoltage of the transistors.

[0011] The present invention employs a precise photomask to pattern apattern-transferred layer (a precise layer), wherein openings aredefined above the channel region of each memory cell and the criticaldimension of these openings is identical. Another patterned photoresistlayer (mask layer) is then formed on the substrate. The patternedphotoresist layer exposes a pre-coding memory cell region in the memorycell region and a gates of the transistors in the periphery circuitregion. Thereafter, using the pattern-transferred layer (precise layer)and the patterned photoresist layer (mask layer) as a coding mask forthe mask read only memory device, the pre-coding memory cell is codedand the threshold voltage of the transistors is adjusted. According tothe present invention, the critical dimension of each of the openings inthe precise layer is about the same and each opening is preciselycorresponded to the channel region of each memory cell. The problem ofhaving critical dimension difference between the coding windows in theisolated pattern region and that in the dense pattern region due to theoptical proximity effect is prevented. Further, when the codeimplantation is being performed on the pre-coding memory cell in thememory cell region, the threshold voltage of the transistors in theperiphery circuit region is concurrently adjusted to further simplifythe manufacturing process.

[0012] Further, the present invention also employs a photoresist layeras the pattern-transferred layer. When the pattern-transferred layercomprises a photoresist material, a deposition step and an etching stepcan be omitted to further simplify the manufacturing process and tofurther reduce the manufacturing cost. Moreover, according to thepresent invention, deviation in the critical dimension between the densepattern region and the isolated pattern region is mitigated by using twophotoresist layers.

[0013] Further, the precise layer in the periphery circuit region can becompletely opened or selectively opened or completely not opened. Thesecond opening and the third opening in the mask layer can form with onepiece of photomask or two pieces of photomask.

[0014] The present invention provides a fabrication method for a readonly memory device. This method provides a substrate that comprises amemory cell region and a periphery circuit region, wherein a memory cellarray is formed in the memory cell region and a plurality of transistorsis formed in the periphery circuit region. A mask layer that comprises aplurality of first openings and second openings is formed on thesubstrate. The first openings are located above a plurality ofpre-coding memory cell regions in the memory cell region and the secondopenings are located above the gates of the transistors in the peripherycircuit region. A precise layer is formed on the memory cell region. Theprecise layer comprises a plurality of third openings, wherein thecritical dimension of each of the third openings is identical and thethird openings are positioned above the channel region of the pre-codingmemory cell in the pre-coding memory cell region. Thereafter, an ionimplantation process is conducted to code the pre-coding memory cells inthe pre-coding memory cell region and to adjust the threshold voltage ofthe transistors, using the mask layer and the precise layer as a codingmask.

[0015] Accordingly, the present invention employs a photomask to patternthe pattern-transferred layer, wherein the pre-coding memory cell regionin the memory cell region and the gates of the transistors in theperiphery circuit region are exposed. Further using the precisephotomask, a plurality of openings having the same critical dimension isdefined in the memory cell region. Further, these openings are locatedabove the channel regions of the pre-coding memory cell in thepre-coding memory cell regions, exposing the word line of the memorycell. The critical dimension of each opening in the precise layer isidentical and each opening in the precise layer precisely corresponds tothe channel region of each memory cell. Even without the application ofthe optical proximity correction technique to manufacture the codingmask, any deviation in the critical dimension of the coding windowbetween the dense pattern region and the isolated pattern region due tothe optical proximity effect is prevented. Further, according to thepresent invention, when the coding implantation is being performed onthe pre-coding memory cells in the memory cell region, the thresholdvoltage of the transistors in the periphery circuit region is beingadjusted concurrently to further simplify the manufacturing process.

[0016] Further, the present invention employs a photoresist layer as apatterned transferred layer. When the pattern-transferred layercomprises a photoresist material, a deposition step and an etching stepcan be omitted to further simplify the manufacturing procedure and tofurther reduce the manufacturing cost. Moreover, the present inventionsimply employs two layers of photoresist to improve any deviation in thecritical dimension between the dense pattern region and the isolatedpattern region.

[0017] Further, the first opening and the second opening of the masklayer can form with one piece of photomask or two pieces of photomask.The precise layer in the periphery circuit region can be completelyopened or selectively opened or completely not opened.

[0018] The present invention provides a fabrication method for a readonly memory device. The method provides a substrate that comprises amemory cell region and a periphery circuit region, wherein a memory cellarray is already formed in the memory cell region and a plurality oftransistors is formed in the periphery circuit region. A negativephotoresist layer is then formed on the memory cell region, followed byperforming a first exposure process to transfer the pattern of the firstphotomask to the first negative photoresist layer, wherein the patternof the first photomask precisely correspondes to the channel region ofeach memory cell in the memory cell region. The non-crosslinked portionof the first photoresist layer is positioned above the channel region ofeach memory cell in the memory cell region. A second negativephotoresist layer is then formed on the substrate. A second exposureprocess is performed to transfer the pattern of the second photomask tothe second photoresist layer and to the first negative photoresistlayer, wherein the pattern of the second photomask corresponds to aplurality of the pre-coding memory cell regions in the memory cellregion and the gates of the transistors in the periphery circuit region.As a result, the non-crosslinked portions of the first negativephotoresist layer and the second photoresist layer are positioned abovethe plurality of the pre-coding memory cell regions in the memory cellregion and above the gates of the transistors in the periphery circuitregion. A development process is conducted to pattern the first negativephotoresist layer and the second photoresist layer. An ion implantationprocess is conducted to code the pre-coding memory cells in thepre-coding memory cell region and to adjust the threshold voltage of thetransistors using the patterned first negative photoresist layer and thepattern second negative photoresist layer as a mask.

[0019] According to the present invention, after forming the firstnegative photoresist layer, the pattern of the precise photomask istransferred to the first negative photoresist layer, wherein thenon-crosslinked portion of the first negative photoresist layer islocated above the channel region of each memory cell. Thereafter, asecond negative photoresist layer is formed. The pattern of the secondphotomask is then transferred to the first negative photoresist layerand the second negative photoresist layer. The opaque portion of thesecond photomask is positioned above the pre-coding memory cell regionin the memory cell region and above the gates of the transistors in theperiphery circuit region. Therefore, the non-crosslinked portions of thefirst negative photoresist layer and the second negative photoresistlayer are positioned above the pre-coding memory cells in the pre-codingmemory cell region. Moreover, the non-crosslinked portion of the secondnegative photoresist layer is also positioned above the gates of thetransistors in the peripheral circuit region. After this, thenon-crosslinked portions of the first negative photresist layer and thesecond photoresist layer are removed to form a plurality of openings inthe memory cell region, wherein the critical dimension of these openingsis identical. Further, these openings are located above the channelregion of each pre-coding memory cell in the pre-coding memory cellregion. Since the critical dimension of each opening above the channelregion of the pre-coding memory cell in the pre-coding memory cellregion is identical, any deviation in the critical dimension of thecoding window between the dense pattern region and the isolated patternregion due to the optical proximity effect is prevented, even withoutthe application of the optical proximity correction technique tomanufacture the coding mask. Moreover, as the code implantation is beingperformed to the pre-coding memory cell in the memory cell region in thepresent invention, the threshold voltage of the transistors in theperiphery circuit region is concurrently being adjusted to furthersimplify the manufacturing process.

[0020] Accordingly, the present invention sequentially performs twoexposure processes on the two negative photoresist layers with twolayers of the photomask and a single development process to pattern thetwo negative photoresist layers. The number of manufacturing steps inthe present invention is greatly reduced to further reduce themanufacturing time and cost. Moreover, the two negative photoresistlayers can be formed into a same photoresist layer. In summary, thepresent invention simply employs two photoresist layers to improve thedeviation in the critical dimension of the coding window between thedense pattern region and the isolated pattern region due to the opticalproximity effect.

[0021] The present invention further provides a fabrication method for aread only memory device. The method provides a substrate that comprisesa memory cell region and a periphery circuit region, wherein a memorycell array is already formed in the memory cell region and a pluralityof transistors is already formed in the periphery circuit region. Afirst negative photoresist layer is formed on the substrate, followed byperforming a first exposure process to transfer the pattern in the firstphotomask to the first negative photoresist layer, wherein the patternin the first photomask corresponds to a plurality of pre-coding memorycells and the gates of the transistors in the periphery circuit region.The non-crosslinked portion of the first negative photoreisist layer ispositioned above a plurality of the pre-coding memory cell regions inthe memory cell region and the gate of the transistors in the peripherycircuit region. The pre-coding memory cell region further comprises theplurality of the pre-coding memory cells. Thereafter, a second negativephotoresist layer is formed on the memory cell region. A second exposureprocess is performed to transfer the pattern of the second photomask tothe first negative photoresist layer and the second negative photoresistlayer, wherein the pattern in the second photomask precisely correspondsto the channel region of each memory cell in the memory cell region. Thenon-crosslinked portions of the second photoresist layer and the firstphotoresist layer are located above the channel region of each memorycell in the pre-coding memory cell region. A development process isperformed to pattern the first negative photoresist layer and the secondnegative photoresist layer. Using the patterned first negativephotoresist layer and the second negative photoresist layer as a mask,an ion implantation process is conducted to code the pre-coding memorycells in the pre-coding memory cell region and to adjust the thresholdvoltage of the transistors.

[0022] According to the present invention, subsequent to the formationof the first negative photoresist layer, the pattern of the firstphotomask is transferred to the first negative photoresist layer. Theopaque portion of the first photomask is located above the pre-codingmemory cell regions in the memory cell region and the gates of thetransistors in the periphery region. Therefore, the non-crosslinkedportion of the first negative photoresist layer in the memory cellregion is located above the pre-coding memory cell regions and the gatesof the transistors. Thereafter, a second negative photoresist layer isformed, followed by transferring the pattern of the second photomask(precise mask) to the first negative photoresist layer and to the secondnegative photoresist layer. The non-crosslinked portion of the firstnegative photoresist layer and the second negative photoresist layer arepositioned above the channel region of each memory cell. After this, thenon-crosslinked portions of the first negative photoresist layer and thesecond negative photoresist layer are removed to form a plurality ofopenings having identical critical dimension. These openings arepositioned above the channel region of each memory cell in thepre-coding memory cell region, exposing the word line of the memorycell. Since the critical dimension of the openings above the channelregion of each pre-coding memory cell is identical deviation in thecritical dimension of the coding window between the dense pattern regionand the isolated pattern region due to the optical proximity effect isprevented, even without the application of the optical proximitycorrection technique. Moreover, as code implantation is performed on thepre-coding memory cells in the memory cell region, the threshold voltageof the transistosr in the periphery circuit region is concurrentlyadjusted to further simplify the manufacturing process.

[0023] Accordingly, the present invention sequentially performs twoexposure processes on the two negative photoresist layers with twolayers of the photomask and a single development process to pattern thetwo negative photoresist layers. The number of manufacturing steps inthe present invention is greatly reduced to further reduce themanufacturing time and cost. Moreover, the two negative photoresistlayers can be formed into a same photoresist layer. In summary, thepresent invention simply employs two photoresist layers to improve thedeviation in the critical dimension of the coding window between thedense pattern region and the isolated pattern region due to the opticalproximity effect.

[0024] Although two photomasks are employed in the manufacturing of aread only memory, one of the photomasks is only required to expose thepre-coding memory cell region in the memory cell region and toconcurrently expose the transistors in the periphery circuit regionduring the coding process. Therefore, this photomask does not have to beas precise as the conventional coding mask. The manufacturing cost forthis photomask is thus lower. The other photomask in this embodiment isa precise mask, wherein the specification is uniform. Although themanufacturing cost for this photomask is higher, this photomask can beused repeatedly because the precise mask is only applicable for formingthe openings having identical critical dimension in the channel regionof the memory cell. Further, the conventional coding mask requires theoptical proximity correction technique to mitigate the inaccuracy inpattern transference due to the optical proximity effect, themanufacturing cost for the conventional coding mask is thus very high.In the present invention, the pattern dimension in the dense patternregion and the pattern dimension in the isolated pattern region areidentical, even without the application of a special patterned photomaskformed with the optical proximity correction technique, themanufacturing cost is thus lower. Further, the advantages provided bythe present invention can be accomplished whether the mask layer isformed before the precise layer or the precise layer is formed beforethe mask layer.

[0025] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0027]FIGS. 1A to 1E are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording a first embodiment of the present invention;

[0028]FIGS. 2A to 2E are schematic top views illustrating the processflow for fabricating a mask read only memory device according to thefirst embodiment of the present invention;

[0029]FIGS. 3A to 3E are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a second embodiment of the present invention;

[0030]FIGS. 4A to 4E are schematic top views illustrating the processflow for fabricating a mask read only memory device according to thesecond embodiment of the present invention;

[0031]FIGS. 5A to 5D are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a third embodiment of the present invention;

[0032]FIGS. 6A to 6D are schematic top views illustrating the processflow for fabricating a mask read only memory device according to thethird embodiment of the present invention;

[0033]FIGS. 7A to 7D are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a fourth embodiment of the present invention; and

[0034]FIGS. 8A to 8D are schematic top views illustrating the processflow for fabricating a mask read only memory device according to thefourth embodiment of the present invention.

DETAILED DESCRIPTION The First Embodiment

[0035]FIGS. 1A to 1E are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording a first embodiment of the present invention. FIGS. 2A to 2Eare schematic top views illustrating the process flow for fabricating amask read only memory device according to the first embodiment of thepresent invention, wherein the memory cell regions in FIGS. 1A to 1E arethe cross-sectional views of FIGS. 2A to 2E along the line I-I″.

[0036] Referring to FIGS. 1A and 2A, a substrate 100 is provided,wherein the substrate 100 is divided into a memory cell region 102 and aperiphery circuit region 104, wherein the memory cell region 102comprises a memory cell array 106, while the periphery circuit region104 comprises a plurality of transistors 108.

[0037] The memory cell array 106 in the memory cell region 102 is formedwith a plurality of gates 106 a, a plurality of bit lines 106 b and aplurality of word lines 106 c. The gates 106 a are disposed on thesubstrate 100. A gate dielectric layer 106 d is further disposed betweenthe gate 106 a and the substrate 100. The bit line 106 b is positionedbetween the gates 106 a in the substrate 100, while the word line 106 ccrosses over the bit line 106 b and electrically connects with the gate106 a. Moreover, the region underneath the gate 106 a and between twobit lines 196 b serves as the channel region 106 e of a memory device.

[0038] The transistors 108 in the periphery circuits region are isolatedfrom each other with isolation structures 109. Each transistor 108 isformed with a gate 108 a, source/drain regions 108 b and a spacer 108 c.The gate 108 a is disposed on the substrate 100, and a gate dielectriclayer 108 d is further disposed between the gate 108 a and the substrate100. The source/drain regions 108 b are located beside the sides of thegate 108 a in the substrate 100. The spacer 108 c is disposed on thesidewall of the gate 108 a.

[0039] Thereafter, a pattern-transferred layer 110 is formed on thesubstrate 100, wherein the pattern-transferred layer 110 is formed witha material that includes silicon oxide, silicon nitride or siliconoxynitride. The pattern-transferred layer 110 is formed by, for example,chemical vapor deposition.

[0040] Referring to FIGS. 1B and 2B, a photoresist layer 112 is formedon the pattern-transferred layer 110. A photolithography process is thenperformed to transfer the pattern of the photomask 114 to thephotoresist layer 112, patterning the photoresist layer 112. Thephotomask 114 is, for example, a precise mask, wherein this mask 114comprises a plurality of patterns with identical critical dimension.Further these patterns are arranged in a matrix. Therefore, the patterndensity in each region of the photomask 114 is identical. Afterpatterning the photoresist layer 112, the photoresist layer 112 in thememory cell region 102 comprises a plurality of openings 116 a. Theseopenings 116 a are disposed above the channel region 106 e of eachmemory cell in the memory cell region 102, exposing a portion of thepattern-transferred layer 110. Concurrently, the patterned photoresistlayer 112 also exposes the pattern-transferred layer 110 in theperiphery circuit region 104.

[0041] Continue to FIGS. 1C and 2C, using the patterned photoresistlayer 112 (as shown in FIGS. 1B and 2B) as an etching mask, an etchingis performed to remove the pattern-transferred layer 110 that is exposedby the openings 116 a to form a plurality of openings 116. Thepattern-transferred layer 110 at the periphery circuit region 104 isalso removed. The openings 116 are positioned above the channel region106 e of each memory cell in the memory cell array 106, exposing aportion of the word line 106 c of the memory cell. The patternedpattern-transferred layer 110 serves as a precise layer, wherein thecritical dimension of each opening 116 in the precise layer are aboutthe same.

[0042] Continuing to FIGS. 1D and 2D, another photoresist layer 118 isformed on the substrate 100 of both the memory cell region 102 and theperiphery circuit region 104. A photolithography process is thenperformed to transfer the pattern of the photomask 120 to thephotoresist layer 118, patterning the photoresist layer 118. Thepatterned photoresist layer 118 in the memory cell region 102 comprisesa plurality of openings 122, wherein these openings 122 expose thepre-coding memory cell region 102 a in the memory cell region 102. Thepre-coding memory cell region 102 a comprises at least one of thepre-coding memory cell to be coded. In other words, the patternedphotoresist layer 118 in the memory cell region 102 covers a portion ofthe openings 116 in the patterned-transferred layer 110 (precise layer)and exposes the openings 116 over the pre-coding memory cell region 102a. Further, the patterned photoresist layer 118 in the periphery circuitregion 104 comprises a plurality of openings 124, wherein these openings124 expose the gates 108 a of the transistors 108. In this embodiment,the pattern-transferred layer 110 (precise layer) and the patternedphotoresist layer 118 serve as the coding mask for the mask read onlymemory device.

[0043] Referring to FIGS. 1E and 2E, an ion implantation process 126 isperformed to code the memory cell by implanting dopants to the channelregion 106 e underneath the gates 106 of each pre-coding memory cell inthe memory cell region 102. Moreover, the threshold voltage adjustmentis also performed on the transistors 108 in the periphery circuit region104.

[0044] As required by the circuit design, an isolated pattern region anda dense pattern region are formed in the memory cell region of the maskread only memory device. However, using a single piece of photomask todefine the coding windows in the isolated pattern region and in thedense pattern region, deviation in the critical dimension between theisolated pattern region and the dense pattern region due to opticalproximity effect is generated. Therefore, according to the aboveembodiment of the present invention, a precise photomask is used topattern the pattern-transferred layer (precise layer) to define theopenings 116 above the channel region 106 e of each memory cell, whereinthe critical dimension of the openings 116 is about identical. Anotherpatterned photoresist layer 118 is further formed on the substrate 100,wherein the patterned photoresist layer 118 exposes the pre-codingmemory cell region 102 a in the memory cell region 102 and thetransistors 108 in the periphery circuit region 104. Thepattern-transferred layer 110 (precise layer) and the patternedphotoresist layer 118 are then served as the coding mask for the maskread only memory device. Since the critical dimension of every opening116 in the precise layer is about identical, deviation in the criticaldimension of the coding window between the dense pattern region and theisolated pattern region due to the optical proximity effect isprevented, even without the application of the optical proximitycorrection technique to manufacture the coding mask.

[0045] Further, two photomasks are used in the above embodiment. Onemask 114 is a precise mask, wherein the specification is uniform.Although the manufacturing cost for such a mask 114 is higher, theprecise mask can be used repeatedly because the precise mask is onlyused to form openings with precisely identical critical dimension overthe channel region of the memory cell. The other photomask 120 that isbeing used in this embodiment is required only to expose the pre-codingmemory cell region in the memory cell region during the coding process.Therefore, the precision of this photomask 120 is not as imperative asthat of the conventional coding mask. Consequently, the manufacturingcost for this photomask 120 is lower. Since the conventional coding maskrequires the application of an optical proximity correction technique toprevent any pattern transference problem created from the opticalproximity effect, the manufacturing cost for the conventional codingmask is very high. In the present invention, the dimension of thepattern in the dense pattern region and in the isolated pattern regionis about the same, even without the application of a special patternedphotomask formed by the application of the optical proximity correctiontechnique, the manufacturing cost is thus reduced. Further, performingthe code implantation to the pre-coding memory cells in the memory cellregion, the threshold voltage of the transistors in the peripherycircuit region is concurrently being adjusted. The manufacturing processis thus greatly simplified.

[0046] Further, in the above first embodiment, a photoresist layer canbe used as the pattern-transferred layer. If the pattern-transferredlayer comprises a photoresist material, a deposition step and an etchingstep can be omitted to further simplify the manufacturing process and tofurther reduce the manufacturing cost. Moreover, according to thepresent invention, simply using two photoresist layers, the deviation inthe critical dimension between the dense pattern region and the isolatedpattern region due to the optical proximity effect is improved.

[0047] Additionally, the precise layer in the periphery circuit regioncan be completely opened or selectively opened or completely not opened.The opening that exposes the pre-coding memory cell region in the memorycell region and the opening that exposes the transistor gates in theperiphery circuit region can be formed with one piece of photomask ortwo pieces of photomasks.

The Second Embodiment

[0048]FIGS. 3A to 3E are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a second embodiment of the present invention. FIGS. 4A to4E are schematic top views illustrating the process flow for fabricatinga mask read only memory device according to the second embodiment of thepresent invention, wherein the memory cell region in FIGS. 3A to 3E arethe cross-sectional views of FIGS. 4A to 4E along the line II-II″.Wherever possible, the same reference numbers are used in the secondembodiment and the first embodiment to refer to the same or like parts.

[0049] Referring to FIGS. 3A and 4A, a substrate 100 is provided,wherein the substrate 100 is divided into a memory cell region 102 and aperiphery circuit region 104. An array of memory cells 106 is alreadyformed in the memory cell region 102, while a plurality of transistors108 is formed in the periphery circuit region 104. The structures of thearray of memory cells 106 and the transistors are same as those in thefirst embodiment and thus will not further reiterate.

[0050] Thereafter, a pattern-transferred layer 210 is formed on thesubstrate 100 of the memory cell region 102 and the periphery circuitregion 104. A material for the pattern-transferred layer 210 is, forexample, silicon oxide, silicon nitride or silicon oxynitride. Thepattern-transferred layer 210 is formed by, for example, chemical vapordeposition.

[0051] Referring to FIGS. 3B and 4B, a photoresist layer 212 is formedon the pattern-transferred layer 210. A photolithography process is thenconducted to transfer the pattern of the photomask 214 to the photoreistlayer 212, patterning the photoresist layer 212. The patternedphotoresist layer 212 in the memory cell region 102 comprises aplurality of openings 216 a, wherein these openings 216 a are locatedabove the pre-coding memory cell region 102 a and these openings 216 aexpose a portion of the surface of the pattern-transferred layer 210.The patterned photoresist layer 212 also comprises a plurality ofopenings 218 a in the peripheral circuit region 104, wherein theseopenings 218 a are positioned above the gates 108 a of the transistors,exposing the surface of the pattern-transferred layer 210.

[0052] Continuing to FIGS. 3C and 4C, an etching is conducted to removethe pattern-transferred layer 210 exposed by the openings 216 a and theopenings 218 a, using the patterned photoresist layer 212 (as shown inFIG. 3B and FIG. 4B) as an etching mask. A plurality of openings 216 isthus formed to expose the pre-coding memory cell region 202 a and aplurality of openings 218 is thus formed to expose the gates 108 a ofthe transistors 108. The patterned pattern-transferred layer 210 is thenserved as a hard mask. Subsequently, the photoresist layer 212 isremoved.

[0053] Referring to FIGS. 3D and 4D, a photoresist layer 220 is formedon the substrate 100. A photolithography process is then performed totransfer the pattern of the photomask 222 to the photoresist layer 220,patterning the photoresist layer 220. The photomask 222 is, for example,a precise mask. The precise mask 222 comprises a plurality of patternshaving identical critical dimension. Further, these patterns arearranged in a matrix. The pattern density in each region on thephotomask 220 is about the same. The patterned photoresist layer 220serves as a precise layer. This precise layer comprises a plurality ofopenings 224, positioned above the channel region 106 e of each memorycell in the pre-coding memory cell region. These openings 224 expose theword line 106 c of the memory cell. Further, the critical dimension ofthe opening 224 is identical. The patterned pattern-transferred layer210 and the patterned photoresist layer 220 (precise layer) serve as thecoding mask for the mask read only memory device.

[0054] Referring to FIGS. 3E and 4E, an ion implantation process 226 isperformed to code the memory cells by implanting dopants to the channelregion 106 e underneath the gate 106 a of the pre-coding memory cells inthe memory cell region 102. Concurrently, the threshold voltage of thetransistor 108 in the periphery circuit region 104 is adjusted tocomplete the code implantation process for the mask read only memorydevice.

[0055] According to the above second embodiment of the presentinvention, a photomask 214 is used to pattern the pattern-transferredlayer 210 to expose the pre-coding memory cell region 102 a in thememory cell region 102. The gates 108 a of the transistors 108 in theperiphery circuit region 104 are also exposed. Further using the precisemask 222 to define a plurality of openings 224 in the memory cellregion, wherein the openings 224 have the same critical dimension. Also,these openings 224 are formed above the channel region 106 e of eachpre-coding memory cell in the pre-coding memory cell region 102 a andexpose the word line 106 c of the memory cell. Since the criticaldimension of each opening 224 in the precise layer is about identicaland the openings 224 precisely correspond to the channel region 106 e ofeach memory cell, the application of the optical proximity correctiontechnique to fabricate the coding photomask is obviated. Accordingly,deviation in the critical dimension of the coding window between thedense pattern region and the isolated pattern region due to the opticalproximity effect is prevented, even without the application of theoptical proximity correction technique to manufacture the coding mask.

[0056] Although two photomasks are employed in the above embodiment, oneof the masks 214 is used only to expose the pre-coding memory cellregion in the memory cell region during the coding process and toconcurrently expose the transistors in the periphery circuit region, theprecision of this mask 214 is not as imperative as that of theconventional coding mask. The manufacturing cost for this mask 214 isthus lower. The other mask 222 in this embodiment is a precise mask,wherein the specification of this mask 222 is uniform. Although themanufacturing cost for this mask 222 in the second embodiment is higher,the mask 222 can be used repeatedly because it is only used for formingopenings 224 with identical critical dimension above the channel regionof the memory cell. Since the conventional coding mask requires theoptical proximity correction technique to prevent any inaccuracy in thetransferred pattern due to the optical proximity effect, themanufacturing cost for the conventional coding mask is very high. Thepattern dimension in the dense pattern region and the isolated patternregion is uniform, even without the application of a special patternedphotomask formed by using the optical proximity correction technique,the manufacturing cost for the present invetnion is lower. Further,performing the coding implantation to the pre-coding memory cells in thememory cell region, the threshold voltage of the transistors in theperiphery circuit region is concurrently being adjusted. Themanufacturing process is thus greatly simplified.

[0057] Further, in the above second embodiment, a photoresist layer canbe used for the pattern-transferred layer. If the pattern-transferredlayer comprises a photoresist material, a deposition step and an etchingstep can be omitted to further simplify the manufacturing process and tofurther reduce the manufacturing cost. Moreover, according to thepresent invention, simply using two layers of the photoresist, thedeviation in the critical dimension between the dense pattern region andthe isolated pattern region due to the optical proximity effect can beimproved.

[0058] Additionally, the precise layer in the periphery circuit regioncan be completely opened or selectively opened or completely not opened.The opening that exposes the pre-coding memory cell region in the memorycell region and the opening that exposes the transistor gate in theperiphery circuit region can be formed with one piece of photomask ortwo pieces of photomask.

Third Embodiment

[0059]FIGS. 5A to 5D are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a third embodiment of the present invention. FIGS. 6A to 6Dare schematic top views illustrating the process flow for fabricating amask read only memory device according to the second embodiment of thepresent invention, wherein the memory cell region in FIGS. 5A to 5D arethe cross-sectional views of FIGS. 6A to 6D along the line III-III″.Wherever possible, the same reference numbers are used in the secondembodiment and the first embodiment to refer to the same or like parts.

[0060] Referring to FIGS. 5A and 6A, a substrate 100 is provided,wherein the substrate 100 is divided into a memory cell region 102 and aperiphery circuit region 104. An array of memory cells 106 is alreadyformed in the memory cell region 102, while a plurality of transistors108 is formed in the periphery circuit region 104. The structures of thearray of memory cells 106 and the transistors 108 are same as those inthe first embodiment and thus will not further reiterate here. Afterthis, a negative photoresist layer 310 is formed on the substrate 100,wherein the negative photoresist layer 310 is formed by, for example,spin coating.

[0061] Referring to FIGS. 5B and 6B, an exposure process is conducted totransfer the pattern of the photomask 312 to the negative photoresistlayer 310, wherein the photomask 312 is a precise photomask. Thephotomask 312 in the memory cell region 102 comprises a plurality ofpatterns having identical critical dimension, wherein these patterns arearranged in a matrix. Thus, the pattern density of the photomask 312 ineach region of the memory cell region 102 is identical. Further, theopaque portion of the photomask 312 is located above the channel region106 e of each memory cell of the memory cell array 106. Thenon-crosslinked portion of the negative photoresist 310 is positionedabove the channel region 106 e of each memory cell and above theperiphery circuit region 104.

[0062] Continuing to FIGS. 5C and 6C, another negative photoresist layer314 is formed on the substrate 100, wherein the photoresist layer 314 isformed by a spin coating method. An exposure process is then performedto transfer the pattern in the photomask 316 to the negative photoresistlayer 314 and the negative photoresist layer 310. The opaque portion ofthe photomask 316 is located above the pre-coding memory cell region 102a of the memory cell region 102 and above the gates 108 a of thetransistors 108 in the periphery circuit region 104. Therefore, thenon-crosslinked portions of the negative photoresist layer 314 and thenegative photoresist layer 310 are positioned above the pre-codingmemory cell region 102 a of the memory cell region 106 and above thegates 108 a of the transistors 108 in the periphery circuit region 104.

[0063] As shown in FIGS. 5D and 6D, a development process is performedto remove the non-crosslinked portions of the photoresist layer 314 andthe photoresist layer 310 to form a patterned negative photoresist layer314 and a patterned negative photoresist layer 310. The patternedphotoresist layer 310 in the pre-coding memory cell region 102 comprisesa plurality of openings 318, wherein these openings 318 are positionedabove the channel region 106 e of the pre-coding memory cell and exposethe word line 106 c of the memory cell. The patterned negativephotoresist layer 314 in the memory cell region 102 comprises aplurality of openings 320, wherein these openings 320 expose thepre-coding memory cell region 102 a. Further, the patterned negativephotoresist layer 314 and the patterned negative photoresist layer 310in the periphery circuit region 104 comprise a plurality of openings322, wherein these openings 322 expose the gates 108 a of thetransistors 108.

[0064] An ion implantation step 324 is then performed to code the memorycells by implanting dopants to the channel region 106 e underneath thegates 106 a in the pre-coding memory cell region of the memory cellregion 102. Moreover, the threshold voltage of the transistors 108 inthe periphery circuit region 104 is concurrently being adjusted tocomplete the coding implantation process for the mask read only memorydevice.

[0065] Subsequent to the formation of the negative photoresist layer 310in the above embodiment of the present invention, the pattern of theprecise mask 312 is transferred to the negative photoresist layer 310,wherein the non-crosslinked portion of the negative photoresist layer310 is positioned above the channel region of each memory cell and abovethe periphery circuit region 104. Further, after the formation of thenegative photoresist layer 314, the pattern of another photomask 316 isthen transferred to the negative photoresist layer 314 and the negativephotoresist layer 310. Therefore, the opaque portion of the photomask316 is positioned above the pre-coding memory cell region 102 a in thememory cell region 102 and above the gates 108 a of the transistors 108in the periphery circuit region 104. The non-crosslinked portions of thenegative photoresist layer 314 and the negative photoresist layer 310are then removed to form a plurality of openings 318 in the pre-codingmemory cell region 102 a, wherein the critical dimension of theseopenings 318 is identical. A plurality of openings 322 is also formed inthe periphery circuit region 104 to expose the gates 108 a of thetransistors 108. The openings 318 are positioned above the channelregion 106 e of each memory cell in the pre-coding memory cell region102 a, wherein the openings 318 expose the word lines 106 c of thememory cell. Since the critical dimension of the openings 318 above thechannel region 106 e of each memory cell in the pre-coding memory cellregion 102 a is identical, deviation in the critical dimension of thecoding window between the isolated pattern region and the dense patternregion due to the optical proximity effect is prevented, even withoutthe application of the optical proximity correction technique to formthe coding mask.

[0066] Further, two photomasks are used in the above embodiment. One ofthe photomasks 312 is a precise mask, wherein the specification isuniform. Although the manufacturing cost for this mask 312 is higher,this mask 312 can be used repeatedly because the precise mask is onlyused for forming openings with precisely identical critical dimensionover the channel region of the memory cell. The other mask 316 that isbeing used in this embodiment is only required to expose the pre-codingmemory cell region in the memory cell region during the coding process.The precision of this mask 316 is not as imperative as that of theconventional coding mask. Consequently, the manufacturing cost for thatother mask 316 is lower. Since the conventional coding mask requires theapplication of the optical proximity correction technique to prevent anyinaccuracy in the transferred pattern due to the optical proximityeffect, the manufacturing cost for the conventional coding mask is thusvery high. In the present invention, the pattern dimension in the densepattern region and the isolated pattern region is about the same, evenwithout the application of a special patterned photomask formed by theoptical proximity correction technique, the manufacturing cost is thusgreatly reduced. Further, as the pre-coding memory cell in the memorycell region is being coded, the threshold voltage of the transistors theperiphery circuit region is being adjusted concurrently to furthersimply the manufacturing process.

[0067] Additionally, the present invention sequentially performs twoexposure processes on the two negative photoresist layers with twolayers of the photomask and a single development process to pattern thetwo negative photoresist layers. Therefore, comparing with the first andthe second embodiments, the third embodiment of the present inventionprovides a more simplified processing procedure to further reduce themanufacturing cost. Moreover, the two negative photoresist layers can beformed into a same photoresist layer. In summary, the present inventionsimply employs two photoresist layers to improve the deviation in thecritical dimension of the coding window between the dense pattern regionand the isolated pattern region due to the optical proximity effect.

Fourth Embodiment

[0068]FIGS. 7A to 7D are schematic cross-sectional views illustratingthe process flow for fabricating a mask read only memory deviceaccording to a third embodiment of the present invention. FIGS. 8A to 8Dare schematic top views illustrating the process flow for fabricating amask read only memory device according to the second embodiment of thepresent invention, wherein the memory cell region in FIGS. 7A to 7D arethe cross-sectional views of FIGS. 8A to 8D along the line IV-IV″.Wherever possible, the same reference numbers are used in the secondembodiment and the first embodiment to refer to the same or like parts.

[0069] Referring to FIGS. 7A and 8A, a substrate 100 is provided,wherein the substrate 100 is divided into a memory cell region 102 and aperiphery circuit region 104. An array of memory cells 106 is alreadyformed in the memory cell region 102, while a plurality of transistorsis formed in the periphery circuit region 104. The structures of thearray of memory cells 106 and the transistors are same as those in thefirst embodiment and thus will not further reiterate here.

[0070] A negative photoresist layer 410 is formed on the substrate 100,wherein the negative photoresist layer 410 is formed by, for example,spin coating.

[0071] Referring to FIGS. 7B and 8B, an exposure process is conducted totransfer the pattern of the photomask 412 to the negative photoresistlayer 410. The opaque portion of the photomask 412 is located above thepre-coding memory cell region 102 a in the memory cell region 102 andabove the gates 108 a of the transistors 108 in the periphery circuitregion 104. In other words, the portion of the negative photoresist 410that are not cross-linked is positioned above the pre-coding memory cellregion 102 a in the memory cell region 102 and above the gates 108 a ofthe transistors 108 in the periphery circuit region 104.

[0072] Thereafter, as shown in FIGS. 7C and 8C, another negativephotoresist layer 414 is formed on the substrate 100, wherein formingthe negative photoresist layer 414 includes the application of thespin-coating method. An exposure process is then conducted to transferthe pattern in the photomask 416 to the negative photoresist layer 414and to the negative photoresist layer 410. The photomask 416 is, forexample, a precise photomask. The photomask 416 comprises a plurality ofpatterns, wherein the critical dimension of these patterns is identical.Moreover, these patterns are arranged in a matrix. The pattern densityin each region of the photomask 416 is about the same. Further, theopaque portion of the photomask 416 is positioned above the channelregion 106 e of each memory cell in the memory cell region 102 a andabove the periphery circuit region 104. In other words, thenon-crosslinked portion of the negative photoresist layer 414 ispositioned above the channel region 106 e of each memory cell and abovethe periphery circuit region 104.

[0073] Referring to FIGS. 7D and 8D, a development process is performedto remove the non-crosslinked portion of the negative photoresist layer414 and the negative photoresist layer 410 to form a patterned negativephotoresist layer 414 and a patterned negative photoresist layer 410.The patterned negative photoresist layer 414 and a patterned negativephotoresist layer 410 in the pre-coding memory cell region 102 acomprise a plurality of openings 418, wherein these openings 418 arepositioned above the channel regions 106 e of each pre-coding memorycell and expose the word lines 106 c of the memory cell. The patternednegative photoresist layer 410 in the periphery circuit region 104comprises a plurality of openings 420, wherein these openings 420 exposethe gates 108 a of the transistors 108.

[0074] Thereafter, an ion implantation process 422 is performed to codethe memory cell by implanting dopants to the channel region 106 e of thepre-coding memory cell in the memory cell region 102, using thepatterned negative photoresist layer 414 and the patterned negativephotoresist layer 410 as a mask. The threshold voltage of thetransistors 108 in the periphery circuit region 104 is concurrentlybeing adjusted to complete the code implantation process of the readonly memory.

[0075] In the above preferred embodiment, subsequent to the formation ofthe negative photoresist layer 410, the pattern of the photomask 412 istransferred to the photoresist layer 410. The opaque portion of thephotomask 412 is positioned above the pre-coding memory cell region 102a in the memory cell region 102 and above the gate 108 a of thetransistor 108 in the periphery circuit region 104. The non-crosslinkedportion of the negative photoresist layer 410 in the memory cell region102 is positioned above the pre-coding memory cell region 102 a, whilethe non-crosslinked portion of the negative photoresist layer 410 in theperiphery circuit region 104 is positioned above the gates 108 a of thetransistors. After this, another negative photoresist layer 414 isformed. The pattern of the photomask 416 (precise mask) is furthertransferred to the negative photoresist layer 414 and the negativephotoresist layer 410. The non-crosslinked portions of the photoresistlayer 414 and the negative photoresist layer 410 are positioned abovethe channel region 106 e of each memory cell. The non-crosslinkedportions of the negative photoresist layer 414 and the negativephotoresist layer 410 are removed to form a plurality of openings 418 inthe pre-coding memory cell region 102 and a plurality of openings 420 inthe periphery circuit region 104, wherein the critical dimension of theopenings 418 is identical and the openings 420 expose the gates 108 a ofthe transistors 108. The openings 418 are positioned above the channelregion 106 e of each memory cell in the pre-coding memory cell region102 a and exposes the word line 106 c of the memory cell. Since thecritical dimension of each opening 418 above the channel region 106 e ofthe pre-coding memory cell in the memory cell region 102 is identical,any deviation in the critical dimension of the coding window between theisolated pattern region and the dense pattern region due to opticalproximity effect is prevented, even without application of the opticalproximity correction technique in forming the coding photomask.

[0076] Further, two photomasks are used in the above embodiment. One ofthe masks is a precise mask, wherein the specification is uniform.Although the manufacturing cost for this mask 416 is higher, this onemask can be used repeatedly because the precise mask is only used forforming openings above the channel region of the memory cell withprecisely identical critical dimensions. The other photomask 412 that isbeing used in this embodiment is required only to expose the pre-codingmemory cell region in the memory cell region during the coding process.The precision of this photomask 412 is not as imperative as that of theconventional coding mask. Consequently, the manufacturing cost for thismask is lower. Since the conventional coding mask requires theapplication of the optical proximity correction technique to prevent anyinaccuracy in the transferred pattern due to the optical proximityeffect, the manufacturing cost for the conventional coding mask is thusvery high. The pattern dimension in the dense pattern region and in theisolated pattern region is identical in the present invention;therefore, even without the application of a specially patternedphotomask formed by the optical proximity correction technique, themanufacturing cost for the present invention is reduced. Further, as thecode implantation is being performed to the pre-coding memory cells inthe memory cell region, the threshold voltage of the transistors in theperiphery circuit region is concurrently being adjusted. Themanufacturing process is thus greatly simplified.

[0077] Accordingly, the present invention sequentially performs twoexposure processes on the two negative photoresist layers with twolayers of the photomask and a single development process to pattern thetwo negative photoresist layers. Therefore, comparing with the first andthe second embodiments, the fourth embodiment of the present inventionprovides a more simplified processing procedure to further reduce themanufacturing cost. Moreover, the two negative photoresist layers can beformed into a same photoresist layer. In summary, the present inventionsimply employs two photoresist layers to improve the deviation in thecritical dimension of the coding window between the dense pattern regionand the isolated pattern region due to the optical proximity effect.

[0078] Moreover, two mask layers are sequentially formed on the memorycell region of the present invention, wherein one of the mask layers isa precise layer. Another mask layer is used to cover the portion of theunwanted pattern on the precise layer to prevent the unwanted pattern totransfer from the precise layer to the underlying material layer.

[0079] Although in the above embodiments, the present invention isdescribed with respect to the fabrication method for a mask read onlymemory device, it is intended that the present invention also applicableto the etching of conductive material in forming the conductive line,the fabrication of trenches or the fabrication of other types ofopening. Additionally, the pattern-transferred layer in the embodimentsof the present invention is a hard mask layer. However, thepattern-transferred layer can be formed with a photoresist material tofurther simply the manufacturing process and cost.

[0080] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. A method for coding a semiconductor device, the method comprising:providing a substrate, the substrate comprises a coding region and aperiphery circuit region, wherein a plurality of coding units are formedin the memory cell region and a plurality of transistors are formed inthe periphery circuit region; forming a precise layer on the codingregion, the precise layer comprises a plurality of first openings,wherein a critical dimension of the first openings is identical and eachof the first openings is disposed above one of the coding units; forminga mask layer on the substrate over the precise layer, the mask layercomprises a plurality of second openings and a plurality of thirdopenings, wherein the second openings are disposed above a plurality ofpre-coding regions and each pre-coding region comprises at least oneselected coding unit, and the third openings are disposed above thegates of the transistors in the periphery circuit region; and performingan ion implantation process to code the at least selected coding unitand to adjust a threshold voltage of the transistors.
 2. The method ofclaim 1, wherein forming the precise layer on the coding region furthercomprises: forming a patterned-transferred layer on the substrate;forming a photoresist layer on the pattern-transferred layer; patterningthe photoresist layer above the coding region with a precise photomaskto form a plurality of fourth openings; and removing thepattern-transferred layer exposed by the fourth openings in the codingregion and the pattern-transferred layer in the periphery circuit regionto form the precise layer in the memory cell region using the patternedphotoresist layer as a mask.
 3. The method of claim 1, wherein formingthe precise layer includes forming a photoresist layer.
 4. The method ofclaim 1, wherein forming the mask layer includes forming a photoresistlayer.
 5. A fabrication method for a read only memory device,comprising: providing a substrate, the substrate comprises a memory cellregion and a periphery circuit region, wherein a memory cell array isformed in the memory cell region and a plurality of transistors isformed in the periphery circuit region; forming a mask layer on thesubstrate, the mask layer comprises a plurality of first openings and aplurality of second openings, the first openings are disposed over atleast one pre-coding memory cell region in the memory cell array,wherein the pre-coding memory cell region comprises at least onepre-coding memory cell, and the second openings are disposed above thetransistor in the periphery circuit region; forming a precise layer onthe memory cell region over the mask layer, the precise layer comprisesa plurality of third openings, each of the third openings is positionedabove one of the pre-coding memory cell in the pre-coding memory cellregion; and performing an ion implantation process to code thepre-coding memory cell in the pre-coding memory cell region and toadjust a threshold voltage of the transistor using the mask layer andthe precise layer as a mask.
 6. The method of claim 5, wherein theforming the mask layer includes forming a photoresist layer.
 7. Themethod of claim 5, wherein forming the mask layer on the substratecomprises: forming a pattern-transferred layer on the substrate; forminga photoresist layer on the pattern-transferred layer; patterning thephotoresist layer; and etching the pattern-transferred layer to form themask layer.
 8. The method of claim 5, wherein forming the precise layeron the memory cell region comprises forming a photoresist layer on thesubstrate.
 9. A fabrication method for a read only memory device,comprising: providing a substrate, the substrate comprises a memory cellregion and a periphery circuit region, wherein a memory cell array isformed in the memory cell region and a plurality of transistors isformed in the periphery circuit region; forming a negative photoresistlayer on the memory cell region; performing a first exposure process totransfer a pattern in a first photomask to the negative photoresistlayer, wherein the pattern in the first photomask corresponds to eachmemory cell in the memory cell region, and a non-crosslinked portion ofthe negative photoresist layer is positioned above the channel region ofeach memory cell in the memory cell region; performing a second exposureprocess to transfer a pattern in a second photomask to the negativephotoresist layer, wherein the pattern of the second photomask preciselycorresponds to at least one pre-coding memory cell region in the memorycell region and the gates of the transistors in the periphery circuitregion, and non-crosslinked portions of the photoresist layer arepositioned above the pre-coding memory cell region in the memory cellregion and above the gates of the transistors in the periphery circuitregion; performing a development process to pattern the negativephotoresist layer; and performing an ion implantation process to thepre-coding memory cell region and to adjust a threshold voltage of thetransistors using the patterned negative photoresist layer as a mask.10. The method of claim 9, wherein the first photomask includes aprecise photomask.
 11. A method for coding a semiconductor device, themethod comprising: providing a substrate, the substrate comprises acoding region and a periphery circuit region, wherein a plurality ofcoding units are formed in the memory cell region and a plurality oftransistors is formed in the periphery circuit region; forming anegative photoresist layer on the substrate; performing a first exposureprocess to transfer a pattern of a first photomask to the negativephotoresist layer, wherein the pattern of the first photomaskcorresponds to at least one pre-coding regions in the coding region andthe gates of the transistors in the periphery circuit region, wherein anon-crosslinked portion of the negative photoresist layer is positionedabove the the pre-coding region in the memory cell region and the gatesof the transistors in the periphery circuit region, and the pre-codingregion comprises at least one coding unit; performing a second exposureprocess to transfer a pattern of a second photomask to the negativephotoresist layer, wherein a pattern of the second photomask preciselycorresponds to the coding units in the coding region, andnon-crosslinked portions of the first negative photoresist layer arepositioned above the coding unit within the pre-coding region;performing a development process to pattern the negative photoresistlayer; and performing an ion implantation process to code the pre-codingunit in the pre-coding region and to adjust a threshold voltage of thetransistors using the patterned negative photoresist layer as a mask.12. The method of claim 11, wherein the second photomask includes aprecise mask.